1. Technical Field
This disclosure generally relates to solid state lighting, for example lighting that employs solid state light sources such a light emitting diodes (LEDs), and in particular drive circuits which supply electrical power to solid state lighting.
2. Description of the Related Art
Solid state lighting has many advantages over traditional lighting, such as incandescent and fluorescent bulbs. Solid state lighting devices exhibit lower energy consumption, longer lifetime, and improved robustness over traditional lighting.
In many solid state lighting applications, a plurality of light emitting diodes (LEDs) are electrically coupled in series configuration, with the same current flowing through all LEDs in the series, thereby insuring that all the LEDs in the “string” have a similar level of brightness or output.
LEDs have a forward voltage, below which little light is emitted. For example, a CREE XP-G type LED has a forward voltage (Vf) of approximately 3.15V. The forward voltage Vf varies considerably between individual LEDs, for example from 2.9V to 3.5V. A series coupled “string” of LEDs therefore has a string forward voltage that is equal to the sum of the forward voltages Vf of the individual LEDs in the string. For example, a string of 16 LEDs might have a forward voltage Vf of 51 Volts. While the forward voltage Vf varies somewhat depending upon the amount of current through the LEDs, this variation is typically small.
Solid state luminaires using LED light sources typically use power converters to convert the mains line voltage into a constant current source to power the LEDs. Switch mode type power converters are typically used to achieve high conversion efficiency.
It is often desirable to design power converters that have an output voltage of 60 volts DC or less in order to comply with the safety low voltage upper limit imposed by safety compliance regulations. Due to component tolerances and open circuit voltage limiting circuitry, it is common to use an LED string with an approximately 50V forward voltage. Commonly available high flux LEDs can dissipate 3 Watts of power, so a 16 LED string typically consumes approximately 48 Watts of power.
There are two options if a higher power lamp or luminaire is desired. A first option is adding a second LED string and second power converter to a lamp or luminaire having a first LED string and respective first power converter. In this option, each LED string is powered via its own respective power converter. A second option is to electrically couple a second string of LEDs in parallel with a first string of LEDs, both strings of LEDs powered via the same power converter. The second option eliminates one power converter so is less costly than the first option.
A drawback of the second option employing parallel strings of LEDs is a “current hogging” effect that results when one LED string has a lower forward voltage Vf than the other parallel string. Unless the LED strings are selected such the respective forward voltages are almost perfectly matched, the current through one LED string will be significantly higher than the current through the other LED string, leading to LED lifetime reduction or even immediate failure. In addition, the LED strings will emit different luminous flux, which may be asthetically objectionable depending upon the design of the luminaire.
Even with careful matching of the string forward voltages of the LED strings, the LED strings must be mounted in such a way as to be isothermal. LED forward voltage Vf is known to vary widely with temperature, so if the LED strings are mounted on different areas of a heat sink, or for some reason have differential cooling, the current in the LED strings will become unbalanced with respect to one another.
Traditional approaches for equalizing current in parallel strings of LEDs have been to use “ballast” resistors, separate linear current regulators for each LED string, and/or careful matching of string forward voltages for the LED strings.
“Ballast” resistors give some electrical isolation between the LED strings but are pure dissipaters, considerably reducing energy efficiency. The addition of linear current regulators, as opposed to switch mode regulators, is somewhat superior to the use of ballast resistors, but still lowers energy efficiency of the luminaire. Forward voltage matching of the LED strings is costly, and often results in high rejection rates of parts. Forward voltage matching also requires that the LED strings be measured at very precisely controlled temperatures, which requires elaborate equipment.
Additionally, it is noted that LEDs may fail in the field because of incorrect soldering, electrostatic discharge or other reasons. In the overwhelming majority of cases the LED fails as a shorted diode. Such reduces the string forward voltage by one LED Vf (e.g., approximately 3 volts for white LEDs). Thus, even if string forward voltages are careful matched during manufacture, in use the string forward voltage may change, leading to the aforementioned problems.
An electrically efficient, easy and inexpensive to manufacture, and/or uncomplicated approach to addressing the aforementioned problems is desirable to make solid state lighting more affordable, robust, and/or aesthetically pleasing.